1. Field of the Invention
The invention relates to field devices and, in particular, to field devices with channel stop regions.
2. Description of the Related Art
High voltage (HV) technology is often used for applications with operating voltages higher than logic or mixed-mode technology. Generally, high voltage technologies are developed based on low voltage technologies. Thus, high voltage devices and low voltage devices are integrated in high voltage technologies. Most local oxidation of silicon (LOCOS) and shallow trench isolation (STI) are developed for isolation between low voltage devices. Accordingly, threshold voltages of the field devices are typically twice higher than operating voltages of low voltage devices such that the field devices are not turned on when the low voltage devices are. In a 1.0 μm logic technology, thickness of the field oxide is typically 6000 Å and threshold voltage of the field device is typically 50V. Moreover, in more advanced technologies, such as 0.35 μm technology, the threshold voltage of the field device is lower because of thinner oxide (about 3500 Å) of the LOCOS or STI structure.
FIG. 1 is a cross section of a conventional metal field device in high voltage technology. The field device comprises a P-well 101, two N-wells 103, a field oxide 105, two N-type doped regions 107, an interlevel dielectric 109, and a metal layer 111. The P-well 101 is separated by the N-wells 103. The field oxide overlays the P-well 101 and the N-wells 103. The N-type doped regions 107 are respectively disposed, in the N-wells 103, on either side of the field oxide 105. The interlevel dielectric 109 overlays the field oxide 105. The metal layer 111 is disposed on the interlevel dielectric 109. In many applications, such as LCD drivers, the high voltages exceed 40V. In other words, a voltage higher than 40V may be applied to the metal layer 111. When the voltage source or process is not optimized, a current path is formed, under the field oxide 105, between the N-type doped regions 107 when a high voltage is applied to the metal layer 111. Thus, the metal field device is erroreously turned on, resulting in malfunction of an integrated circuit.